Catalysts containing tungstate for the synthesis of alkylmercaptane and method for the production thereof

ABSTRACT

The invention relates to a catalyst containing alkali tungstate for the synthesis of alkylmercaptanes from alkanols and hydrogen sulphide, in addition to a method for the production of said catalyst, wherein the molar ratio of alkali to tungstan is &lt;2:1.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application claims the benefit of priority of German ApplicationNo. 10 2004 037 739.1 filed Aug. 4, 2004, and International ApplicationNo. PCT/EP2005/007211 filed Jul. 5, 2005, which are relied on andincorporated herein by reference.

INTRODUCTION AND BACKGROUND

The present invention relates to a catalyst comprising alkali metaltungstate for the synthesis of alkyl mercaptans from alkanols andhydrogen sulphide, and to a process for preparing this catalyst.

In this patent application, the term alkali is understood to mean thebound alkali metals of the Periodic Table of the Elements or mixtures ofat least two alkali metals bound in the tungstates. In this case,caesium occurs only together with a further element of the alkali metalgroup.

Methyl mercaptan in particular is an industrially importantintermediate, for example for the synthesis of methionine and for thesynthesis of dimethyl sulphoxide and dimethyl sulphone. It is nowadaysprepared predominantly from methanol and hydrogen sulphide by reactionover a catalyst composed of aluminium oxide. The methyl mercaptan issynthesized commonly in the gas phase at temperatures between 300 and500° C. and at pressures between 1 and 25 bar.

In addition to the methyl mercaptan formed, the reaction mixturecomprises the unconverted starting materials and by-products, forexample dimethyl sulphide and dimethyl ether, and also the gases whichare inert for the purposes of the reaction, for example methane, carbonmonoxide, hydrogen and nitrogen. The methyl mercaptan formed is removedfrom this reaction mixture.

For the economic viability of the process, a maximum selectivity isrequired in the catalytic reaction of methanol and hydrogen sulphide togive methyl mercaptan in order to keep the removal of the methylmercaptan formed from the reaction mixture as uncomplicated andinexpensive as possible. Here, especially the energy demands for thecooling of the reaction gas mixture to condense the methyl mercaptanconstitute a large cost factor.

To increase activity and selectivity, aluminium oxide as a support istypically admixed with potassium tungstate or caesium tungstate. In thiscase, the tungstate is commonly used in amounts up to 25% by weightbased on the total weight of the catalyst. An improvement of activityand selectivity is also obtained by increasing the molar ratio ofhydrogen sulphide to methanol. Typically, molar ratios between 1 and 10are employed.

However, a high molar ratio also means a high excess of hydrogensulphide in the reaction mixture and thus the need to conduct largeamounts of gas in circulation. To reduce the energy demands required forthis purpose, the ratio of hydrogen sulphide to methanol shouldtherefore deviate only slightly from 1.

U.S. Pat. No. 2,820,062 relates to a process for preparing organicthiols, in which a catalyst composed of active aluminium oxide which isadmixed with potassium tungstate in an amount of 1.5 to 15% by weight,based on the weight of the catalyst, is used. With this catalyst, goodactivities and selectivities are achieved at reaction temperatures of400° C. and molar ratios of 2. This US patent mentions variouspossibilities for the introduction of the potassium tungstate into thealuminium oxide. For instance, it is said to be possible to employimpregnation processes, coprecipitations and pure mixtures. Littlesignificance is attributed to the actual preparation of the catalyst forthe economic viability of the synthesis process of methyl mercaptan.

EP 0 832 687 B1 describes the advantages of the use of caesium tungstate(Cs₂WO₄) instead of potassium tungstate (K₂WO₄) as a promoter. Forinstance, use of caesium tungstate can achieve an enhanced activity withsimultaneously good selectivity.

Increasing the caesium tungstate concentration to up to 40% by weightallows the selectivity for methyl mercaptan to be increased to 92%without the activity being disproportionately worsened.

According to the general view, the best selectivity is achieved withcatalysts for which the alkali metal/tungsten ratio is equal to 2:1 (A.V. Mashkina et al., React. Kinet. Catal. Lett., Vol. 36, No. 1, 159-164(1988).

SUMMARY OF INVENTION

It is an object of the present invention to specify a catalyst and aprocess for its preparation, which, at low molar ratios of hydrogensulphide to methanol, features improved activity and selectivitycompared to the known catalysts and thus leads to better economicviability of the process.

This object is achieved by the provision of a catalyst comprising acatalytically active alkali metal tungstate which contains bound alkalimetals and tungsten with a molar ratio of alkali metals to tungsten of<2:1, in particular of <2:1 to 0.9:1, preferably 1.9:1 to 1:1,particularly 1.6:1 to 1:1.

The oxidic composition can be described with the formula A_(x)WO_(y) inwhich A is alkali metal and x is <2 to 0.9 and y is 3.4 to <4.

The bound alkali metal constituent of the tungstate can be composed ofone or more elements of the alkali metal group. In this case, caesiumoccurs only in combination with another alkali metal element.

The catalyst contains the tungstate in an amount of 8 to 45% by weight,in particular 15 to 36% by weight, preferably >25 to 36% by weight. Inthe case of a coated catalyst, these proportions are based on thecomposition of the coating.

The oxidic compounds composed of alkali metal(s) and tungsten may beimpregnated directly onto a support body (supported catalyst).

In the case of the preparation of catalysts in the form of extrudates ormouldings, the pulverulent support is impregnated or mixed with theoxidic composition and the resulting intermediate is subsequentlyreshaped (unsupported catalyst). When a coated catalyst is prepared, thepulverulent support is impregnated with the catalytically activecomposition and the resulting mixture is then applied to a preferablyinert support core in the form of a coating.

The alkali metal/W ratio preferably ranges from <1.9:1 to 1:1. Theinventive catalysts for the reaction of alkanols with hydrogen sulphideto give alkyl mercaptans thus comprise a superstoichiometric proportionof tungsten compared to the catalyst impregnated with caesium tungstate(Cs₂WO₄) or potassium tungstate (K₂WO₄) according to the prior art.

It is found that this higher proportion in the tungstate on thealuminium oxide used with preference in comparison to the stoichiometricalkali metal tungstate used exclusively in the prior art imparts to thecatalyst an improved activity with simultaneously improved selectivity.While the increase in the concentration of caesium tungstate (Cs₂WO₄) onthe catalyst merely brings about an increase in the selectivity withsimultaneously lower activity, a further increase in the selectivitywith simultaneously increased activity is unexpectedly found in the caseof the increase in the tungsten content in relation to the alkali metalcontent. According to the invention, an excellent activity can beachieved at very high loadings with the promoter without the activity ofthe catalyst, as known from the prior art, decreasing. In addition, ithas also been found that the activity and selectivity of the catalystcan be adjusted precisely via the alkali metal-tungsten ratio and viathe selection of the alkali metals. When mixtures of alkali metals areused, it is additionally possible to replace the comparatively moreexpensive metals such as caesium or rubidium at least partly with lessexpensive metals, for example potassium or sodium, without the activityor selectivity of the catalyst being impaired.

DETAILED DESCRIPTION OF INVENTION

The catalyst is used in the form of a supported catalyst in which thesurface is impregnated with the catalytically active substance, or of acoated catalyst in which a preferably inert core is surrounded with amixture of catalytically active substance and support material. Inaddition, extrudates or mouldings in which the catalytically activesubstance is mixed with the pulverulent support material before thereshaping or is impregnated with it may be used. The support materialsused are the known oxidic inorganic compounds, for example SiO₂, TiO₂,ZrO₂ and preferably what is known as active aluminium oxide. Thismaterial has high specific surface areas between about 10 and 400 m²/gand consists mainly of oxides of the transition series of thecrystallographic phases of aluminium oxide (see, for example, Ullmann'sEncyclopaedia of Industrial Chemistry of 1985, Vol. A1, pages 561-562).These transition oxides include γ-, δ-, η-, κ-, χ- and θ-aluminiumoxide. All of these crystallographic phases are converted on heating ofthe aluminium oxide to temperatures above 1100° C. to the thermallystable α-aluminium oxide. Active aluminium oxide is suppliedcommercially for catalytic applications in various qualities and supplyforms. Particularly suitable for the preparation of supported catalystsare support bodies composed of granulated or extruded aluminium oxidehaving particle diameters of 1 to 5 mm, a specific surface area of 180to 400 m²/g, a total pore volume between 0.3 and 1.2 ml/g, and a bulkdensity of 300 to 900 g/l. For the purposes of the invention, preferenceis given to using aluminium oxide having a specific surface area of morethan 200 m²/g, since the catalytic activity of the finished catalystrises slightly with increasing surface area of the aluminium oxide. Thismaterial is used in powder form preferably for the preferred of thecoated catalysts, extrudates or mouldings.

The aqueous impregnation solution for the application of the promotercan be prepared in a simple manner from water-soluble alkali metal andtungsten compounds, in particular tungstic acid (H₂WO₄) and alkali metalhydroxides. To this end, for example, tungstic acid is suspended inwater and dissolved with addition of a base and heating. Alkali metalhydroxide or another alkali metal salt is likewise dissolved in waterand combined with the solution of tungstic acid (promoter solution).Also advantageously usable are alkali metal salts whose anions can bedriven out without residue by heat treatment, for example nitrates,formates, oxalates, acetates or carbonates. Suitable for stabilizingthis solution having a pH of 8 to 14 are inorganic and also organicbases. Preference is given to using those bases which can be driven outwithout residue by a final heat treatment of the catalyst obtained afterthe impregnation. These bases preferably include ammonium hydroxide andorganic bases, in particular amines. Compared to the prior art, themolar ratio of alkali metals and W when the aqueous impregnationsolution is prepared is selected in such a way that, in contrasts tocaesium tungstate (Cs₂WO₄) or potassium tungstate (K₂WO₄) having analkali metal/W ratio of 2 to 1, a higher proportion of tungsten, i.e. analkali metal to W ratio of less than 2 to 1, in particular <1.9:1 to0.9:1, is present. In comparison to the known catalysts, this leads to adistinctly increased activity and selectivity of the inventivecatalysts, in particular at low ratios of hydrogen sulphide and methanolin the reaction gas.

When mixtures of tungstates with mixed alkali metal fractions are used,they are preferably two different alkali metals of the Periodic Table ina ratio between 0.01:1.0 and 1.0:1.0. In this case, the proportion ofthe less expensive alkali metal is preferably increased to such anextent and simultaneously that of the comparatively more expensivealkali metal reduced in return that no deterioration in the activity orselectivity of the catalyst occurs.

For the application of the promoter solution, various impregnationtechniques, such as immersion impregnation, spray impregnation, vacuumimpregnation and pore volume impregnation may be used, and theimpregnation may also be effected repeatedly. In the case of mouldings,the selected impregnation process has to enable the desired loadingamount of the promoter to be applied with good uniformity over the totalcross section.

The promoter solution is preferably applied to the shaped bodies byspray or vacuum impregnation in one or two steps. In spray impregnation,the aqueous impregnation solution is sprayed onto the support bodies. Invacuum impregnation, a reduced pressure is generated by means of avacuum pump in a vessel charged with the shaped bodies. Opening of ahose connection to the aqueous impregnation solution sucks the solutioninto the vessel until the entire bed of shaped bodies is covered withthe solution. After an impregnation time of 0.2 to 2 hours, the solutionwhich has not been absorbed by the material is drained off or pouredoff.

Predrying at room temperature for a period of 1 to 10 hours allows theinitial concentration gradient over the cross section of the shapedbodies to be substantially balanced. Thus, the uniformity of theimpregnation over the cross section of the catalyst particles isimproved. Preference is given to drying the thus obtained catalystprecursors to remove the residual moisture at 100 to 200° C., preferably100 to 140° C., for the period of 1 to 10 hours. There is then acalcination at 300 to 600° C., preferably 420 to 480° C., for the periodof 1 to 20 hours, preferably 1 to 5 hours. This fixes the promoter onthe aluminium oxide and decomposes and drives off the base of theimpregnation solution. Optionally, the bed of support bodies of thecatalyst precursors can be flowed through by a gas stream in the courseof the predrying, drying and calcinations, which improves the removal ofthe residual moisture and of the decomposition gases.

The shaped bodies can also be impregnated in a plurality of stages, inparticular two stages.

In a preferred embodiment, the solution used in the first stage thencomprises one to two thirds of the intended total amount of alkali metaland tungsten compounds.

When the procedure has a plurality of stages, but at least two stages,the precursor obtained in the first step is optionally not calcined.

Otherwise, the same impregnation, drying and calcination programme asdescribed for the one-stage process proceeds in the second stage.

This multistage impregnation is viable in particular when high loadingsare desired and/or the limited solubility of the promoter mixture doesnot enable the loading in one step.

The possibility also exists of spraying the support bodies repeatedlywith the impregnation solution during the impregnation operation (step afrom claim 11) and, between these treatment steps, in each case removingportions of the residual moisture at a temperature of up to 120° C.,before moving on to step b.

In the preparation of the coated catalyst, the powder to be applied as acoating may be calcined before or after the coating. For example, thiscatalyst type may be prepared according to EP-B-0 068 193. In thepreparation of the extrudates or of the mouldings too, the calcinationsmay be effected before and/or after the reshaping.

EXAMPLES Example 1 (Comparative Example)

150 g of aluminium oxide I were impregnated with 21.0% by weight ofcaesium tungstate (Cs_(2.0)WO₄) with the aid of vacuum impregnation. Tothis end, the specific procedure was as follows:

To prepare the impregnation solution, 55.7 g of tungstic acid weresuspended in 44.5 g of water and dissolved by adding 111.4 g of 25%ammonia solution and heating to 50° C. 74.6 g of Cs(OH).H₂O weredissolved in 37.3 g of water and mixed with the first solution. Thesolution was subsequently stirred in a covered beaker for 48 hours.Thereafter, the solution was made up to a volume of 234 ml with 25 g ofwater.

The aluminium oxide was initially charged in a glass vessel which wasevacuated to 150 mbar. By virtue of the opening of a tap, theimpregnation solution was sucked into the evacuated glass vessel untilthe entire bed of shaped bodies was covered with the solution. After await time of 15 minutes and aeration of the glass vessel, the solutionwhich had not been absorbed by the aluminium oxide ran back into thebeaker. 79 ml of impregnation solution were absorbed by the aluminiumoxide.

The granules were dried to remove the residual moisture at roomtemperature in an air current for the period of 1 hour and subsequentlyat 120° C. for 3 hours. Afterward, the granules were calcined at 455° C.for 3 hours.

Example 2 (Comparative Example)

Comparative Example 1 was repeated with 26.3% loading of the aluminiumoxide with caesium tungstate (Cs_(2.0)WO₄).

Example 3 (Comparative Example)

Comparative Example 1 was repeated with 19.6% loading of the aluminiumoxide with potassium tungstate (K_(2.0)WO₄) with use of KOH instead ofCs(OH).H₂O.

Example 4

150 g of aluminium oxide (Spheralite 501A) was impregnated in atwo-stage impregnation with a total of 26.7% by weight of promoter(K_(1.6)WO_(y)) with the aid of vacuum impregnation. The specificprocedure was as follows:

64.5 g of tungstic acid were suspended in 50.7 g of water and dissolvedby adding 126.9 g of 25% ammonia solution and heating to 50° C. 22.8 gof KOH were dissolved in 11.5 g of water and mixed with the firstsolution. The solution was subsequently stirred in a covered beaker for48 hours. Thereafter, the solution was made up to a volume of 234 mlwith 39 g of water. The aluminium oxide was initially charged in a glassvessel which was evacuated to 150 mbar. By virtue of the opening of atap, the impregnation was sucked in until the entire bed of mouldingswas covered with the solution. After a wait time of 15 minutes andaeration of the glass vessel, the solution which had not been absorbedby the aluminium oxide flowed back into the beaker. 76 ml ofimpregnation solution were absorbed by the aluminium oxide.Subsequently, the granules were dried at room temperature for 1 hour andat 120° C. for 3 hours, and calcined at 455° C. for 3 hours.

To carry out the second impregnation, an identical impregnation solutionto that in the first step was prepared and applied in the same way byvacuum impregnation to the already laden catalyst from the first step.This was then followed again by drying at room temperature for 1 hour,followed by drying at 120° C. for 3 hours. Finally, the catalystparticles were calcined under air at 455° C. for 4 hours.

Example 5

150 g of aluminium oxide (Spheralite 501A) was impregnated in atwo-stage impregnation with a total of 30.1% by weight of promoter(Rb_(0.9)WO_(y)) with the aid of vacuum impregnation. The specificprocedure was as follows:

59.0 g of tungstic acid were suspended in 48.3 g of water and dissolvedby adding 110.7 g of 25% ammonia solution and heating to 50° C. 41.5 gof RbOH were dissolved in 17.5 g of water and mixed with the firstsolution. The solution was subsequently stirred in a covered beaker for48 hours. Thereafter, the solution was made up to a volume of 234 mlwith 25 g of water. The aluminium oxide was initially charged in a glassvessel which was evacuated to 150 mbar. By virtue of the opening of atap, the impregnation was sucked in until the entire bed of mouldingswas covered with the solution. After a wait time of 15 minutes andaeration of the glass vessel, the solution which had not been absorbedby the aluminium oxide flowed back into the beaker. 75 ml ofimpregnation solution were absorbed by the aluminium oxide.Subsequently, the granules were dried at room temperature for 1 hour andat 120° C. for 3 hours, and calcined at 455° C. for 3 hours.

To carry out the second impregnation, an identical impregnation solutionto that in the first step was prepared and applied in the same way byvacuum impregnation to the already laden catalyst from the first step.This was then followed again by drying at room temperature for 1 hour,followed by drying at 120° C. for 3 hours. Finally, the catalystparticles were calcined under air at 455° C. for 4 hours.

Example 6

150 g of aluminium oxide (Spheralite 501A) was impregnated in atwo-stage impregnation with a total of 29.4% by weight of promoter(K_(0.7)CS_(0.7)WO_(y)) with the aid of vacuum impregnation. Thespecific procedure was as follows:

61.3 g of tungstic acid were suspended in 49.1 g of water and dissolvedby adding 122.7 g of 25% ammonia solution and heating to 50° C. 9.8 g ofKOH and 29.0 g of Cs(OH).H₂O were dissolved in 14.5 g of water and mixedwith the first solution. The solution was subsequently stirred in acovered beaker for 48 hours. Thereafter, the solution was made up to avolume of 234 ml with 47 g of water. The aluminium oxide was initiallycharged in a glass vessel which was evacuated to 150 mbar. By virtue ofthe opening of a tap, the impregnation was sucked in until the entirebed of mouldings was covered with the solution. After a wait time of 15minutes and aeration of the glass vessel, the solution which had notbeen absorbed by the aluminium oxide flowed back into the beaker. 75 mlof impregnation solution were absorbed by the aluminium oxide.Subsequently, the granules were dried at room temperature for 1 hour andat 120° C. for 3 hours, and calcined at 455° C. for 3 hours.

To carry out the second impregnation, an identical impregnation solutionto that in the first step was prepared and applied in the same way byvacuum impregnation to the already laden catalyst from the first step.This was then followed again by drying at room temperature for 1 hour,followed by drying at 120° C. for 3 hours. Finally, the catalystparticles were calcined under air at 455° C. for 4 hours.

Example 7

150 g of aluminium oxide (Spheralite 501A) was impregnated in atwo-stage impregnation with a total of 31.0% by weight of promoter(Na_(0.3)Cs_(1.1)WO_(y)) with the aid of vacuum impregnation. Thespecific procedure was as follows:

61.1 g of tungstic acid were suspended in 48.9 g of water and dissolvedby adding 122.1 g of 25% ammonia solution and heating to 50° C. 3.2 g ofNaOH and 44.6 g of Cs(OH).H₂O were dissolved in 22.3 g of water andmixed with the first solution. The solution was subsequently stirred ina covered beaker for 48 hours. Thereafter, the solution was made up to avolume of 234 ml with 40 g of water. The aluminium oxide was initiallycharged in a glass vessel which was evacuated to 150 mbar. By virtue ofthe opening of a tap, the impregnation was sucked in until the entirebed of mouldings was covered with the solution. After a wait time of 15minutes and aeration of the glass vessel, the solution which had notbeen absorbed by the aluminium oxide flowed back into the beaker. 74 mlof impregnation solution were absorbed by the aluminium oxide.Subsequently, the granules were dried at room temperature for 1 hour andat 120° C. for 3 hours, and calcined at 455° C. for 3 hours.

To carry out the second impregnation, an identical impregnation solutionto that in the first step was prepared and applied in the same way byvacuum impregnation to the already laden catalyst from the first step.This was then followed again by drying at room temperature for 1 hour,followed by drying at 120° C. for 3 hours. Finally, the catalystparticles were calcined under air at 455° C. for 4 hours.

Example 8 (Use Example)

The catalysts were tested with regard to their performance data in thesynthesis of methyl mercaptan from hydrogen sulphide and methanol.

The synthesis was carried out in a stainless steel tube of internaldiameter 18 mm and a length of 500 mm. The catalyst bed of in each case76 ml was secured in the reaction tube on both sides by inert beds ofglass spheres. The reaction tube was heated to the reaction temperatureof about 320° C. using a jacket comprising a thermal oil.

The experimental conditions can be taken from the following list:

GHSV: 1300 h⁻¹ (based on standard conditions) LHSV: 0.84 h⁻¹ (based onliquid MeOH) Reaction temperature: 320° C. Mass ratio 1.9 H₂S/MeOH:Pressure: 9 bar

The reaction mixture comprising the products methyl mercaptan, dimethylsulphide and dimethyl ether, and comprising the unconverted startingmaterials methanol and hydrogen sulphide is analyzed by online gaschromatography.

When the tungsten fraction in relation to the alkali metal fraction inthe catalyst is increased, a distinct increase in activity can be seenwith simultaneously improved selectivity. In comparison to the priorart, this leads to a yield increase of up to 10%. The selectivity can beincreased to up to ˜96.5% by adjusting the alkali metal-tungstate ratio,and the methanol conversion rises. In the industrial scale synthesis ofmethyl mercaptan, this also leads to considerable cost savings in theremoval of the reaction products from unconverted methanol andby-products.

In addition, the results of Examples 4 to 7 show that at least a portionof the alkali metals can be exchange for one another in order toselectively adjust the activity and selectivity of the catalyst or inorder to save raw material costs in the catalyst synthesis.

TABLE 1 Experimental results mol. alkali Loading Methanol CatalystAlkali metal:W [% by conversion Selectivity Yield Example metal ratiowt.] [%] [%] [%] CE1 Cs   2:1 21.0 82.4 93.3 76.9 CE2 Cs   2:1 26.3 79.594.7 75.2 CE3 K   2:1 19.6 76.0 95.2 72.4 E4*⁾ K 1.6:1 26.7 85.6 95.181.4 E5*⁾ Rb 0.9:1 30.1 73.2 96.6 70.7 E6*⁾ K, Cs 1.4:1 29.4 88.5 95.484.4 E7*⁾ Na, Cs 1.4:1 31.0 88.4 95.8 84.7 CE1: Catalyst according toComparative Example 1 *⁾multistage impregnation

1. A supported catalyst comprising a support material and acatalytically active tungstate which contains at least two chemicallybound alkali metals which are different elements and tungsten in a molarratio of the sum of the alkali metals to tungsten of <2:1, cesiumoccurring only in combination with another alkali element.
 2. Thesupported catalyst according to claim 1, consisting of a coated catalystin which a support core is coated with the catalytically activetungstate with a support material impregnated with this tungstate. 3.The supported catalyst according to claim 1, in which the supportmaterial impregnated with the catalytically active tungstate has beenprocessed to give an unsupported catalyst.
 4. The supported catalystaccording to claim 1, in which the surface of a support body isimpregnated with a catalytically active oxidic composition composed ofalkali metals and tungsten with a molar ratio of alkali metals totungsten of <2:1.
 5. The supported catalyst according to claim 1, inwhich the molar ratio of alkali metals to tungsten in the tungstate is<2:1 to 0.9:1.
 6. The supported catalyst according to claim 5, in whichthe ratio ranges from 1.9:1 to 1:1.
 7. The supported catalyst accordingto claim 1, in which a catalytically active oxidic compositioncorresponds to the general formulaA_(x)WO_(y) in which A: is at least two alkali metal x: is 0.9 to <2 forthe sum of at least two alkali metals y: is 3.4 to <4.


8. The supported catalyst according to claim 1, comprising tungstate inan amount of 8 to 45% by weight, preferably 20 to 36% by weight.
 9. Thesupported catalyst according to claim 1, in which the support materialconsists of an oxidic inorganic compound.
 10. The supported catalystaccording to claim 9, in which the support material consists of aluminumoxide (Al₂O₃).
 11. The supported catalyst according to claim 9, in whichthe support material has a specific surface area of 180 to 400 m²/g(BET) and a total pore volume of 0.3 to 1.2 ml/g.
 12. The supportedcatalyst according to claim 1, in which the alkali metals are potassiumand cesium.
 13. The supported catalyst according to claim 1, in whichthe alkali metals are sodium and cesium.
 14. The supported catalystaccording to claim 1, in which the alkali metals are rubidium andcesium.
 15. The supported catalyst according to claim 1, in which thealkali metals are sodium and potassium.
 16. The supported catalystaccording to claim 1, in which the alkali metals are rubidium andpotassium.
 17. A process for preparing a supported catalyst comprising asupport material and an alkali metal tungstate comprising: a)impregnating the support material with an aqueous solution whichcomprises soluble alkali metal and tungsten compounds in the desiredmolar alkali metal/tungsten ratio, b) predrying of the resultingimpregnated shaped body or of the finely divided support material(catalyst precursor) at room temperature, c) optional drying at 100 to200° C. to remove the residual moisture, d) subsequent calcining attemperatures of 300 to 600° C. for the period of 2 to 10 hours, e)obtaining the supported catalyst or the impregnated finely dividedsupport material with a content of 8 to 45% by weight of a promoter ofthe general compositionA_(x)WO_(y) in which A is at least two alkali metals which are differentelements, x is <2 for the sum of at least two alkali metals which aredifferent elements and y is <4, and f) suspending the finely dividedimpregnated support material with addition of known assistants andapplied to an inert support core or extruded and compressed.
 18. Theprocess according to claim 17, wherein steps a to c, and optionally d,are repeated at least once.
 19. The process according to claim 17, suchthat in the case of repeated impregnation, the impregnation solutionused first comprises one to two thirds of the intended total amount ofalkali metals and tungsten.
 20. The process according to claim 17, suchthat the support material is sprayed repeatedly with the impregnationsolution and portions of the residual moisture are removed at atemperature up to 120° C. between these treatment steps, before movingon to treatment step b).
 21. The process according to claim 17, suchthat the catalyst is heat-treated after the application of theimpregnated support material to the core after the extrusion orcompression step.
 22. The process according to claim 1 for preparingalkyl mercaptans by reacting alkanols and hydrogen sulphide in thepresence of the catalyst.
 23. The process according to claim 22 forpreparing methyl mercaptan by reacting methyl alcohol and hydrogensulphide.
 24. The process according to claim 17, such that the contentof the promoter is 15 to 36% by weight.